Laser flash absorption spectroscopy study of ferredoxin reduction by photosystem I in Synechocystis sp. PCC 6803: evidence for submicrosecond and microsecond kinetics.

The kinetics of reduction of soluble ferredoxin by photosystem I (PSI), both purified from the cyanobacterium Synechocystis sp. PCC 6803, were investigated by flash-absorption spectroscopy between 460 and 600 nm. Most experiments were made with isolated monomeric PSI reaction centers prepared with the detergent beta-dodecyl maltoside. Analysis of absorption transients, in parallel at 480 and 580 nm and under several conditions, shows the existence of three different first-order components in the presence of ferredoxin (t1/2 approximately 500 ns, 20 microseconds, and 100 microseconds). A second-order phase of ferredoxin reduction is also present [k = (2-5) x 10(8) s-1 at pH 8 and at moderate ionic strength]. Similar first-order kinetic components were found with membranes from Synechocystis, with dissolved crystals of trimeric PSI reaction centers from Synechococcus, and also when ferredoxin from Synechocystis is replaced by ferredoxin from Chlamydomonas reinhardtii. The three first-order phases exhibit similar, though not identical, spectra which are consistent with electron transfer from the [4Fe-4S] centers of PSI to the [2Fe-2S] center of ferredoxin and are all attributed to reduction of ferredoxin bound to PSI. At pH 8 and at moderate ionic strength, the dissociation constants associated with each of these components are also similar, with a global value varying between 0.2 and 0.8 microM in different cyanobacterial preparations. The presence of three exponential components is discussed assuming homogeneity of the two partners and using the estimated values for the shortest possible distance of approach of soluble ferredoxin from the different iron-sulfur centers of PSI. It is concluded that the 500-ns phase corresponds to electron transfer from either FA- or FB-, the terminal iron-sulfur acceptors of PSI, to ferredoxin and that the immediate electron donor to ferredoxin is reduced within less than 500 ns. The presence of at least two different types of PSI-ferredoxin complex, all competent in electron transfer, is also deduced from the kinetic behavior.